You are right, in fact it’s still possible to study datafag, I stand corrected: https://uit.no/utdanning/program/795103/informatikk_datafag_-_bachelor

With regards to “sluttfag”, I couldn’t find anything real, but Google’s AI summary says “Et sluttfag er et fag på videregående skole som blir avsluttet med en standpunktkarakter det gjeldende skoleåret” - so it’s the final year of a subject that gives you a grade on your high-school diploma… according to a language model :l